Low temperature bump-bonding semiconductor imaging device

ABSTRACT

A semiconductor imaging device, for use, for example, in medical diagnosis and non-destructive testing, includes a radiation detector semiconductor substrate and a readout substrate connected to the detector by means of low temperature solder bumps A low temperature solder is preferably a lead-tin based solder having a melting point below that of eutectic lead-tin solder. Preferred embodiments of such low temperature solder include bismuth based alloys such as, for example, the eutectic (52 wt-%Bi, 32 wt-%Pb, 16 wt-%Sn) alloy which has a melting point under 100° C.

FIELD OF THE INVENTION

The invention relates to an imaging device comprising a detectorsubstrate including a plurality of detector cells bump-bonded to areadout substrate including a corresponding plurality of readout cellsand to a method of manufacturing such an imaging device.

BACKGROUND OF THE INVENTION

Examples of semiconductors used for imaging devices are: CdZnTe, Si,CdTe, HgI₂, InSb, GaAs, Ge, TiBr, PbI₂.

A detector substrate may comprise a plurality of detector cells (e.g.,pixel cells) defined by metal contacts on one side of the detector. Thereadout substrate can comprise a corresponding plurality of readoutcircuits or charge coupled device (CCD) cells. The readout substrate canbe bump-bonded to the detector substrate with individual pixel cellsbeing connected to corresponding readout circuits or CCD cells byrespective conductive bumps.

Imaging devices of this type can be used for medical applicationsinvolving the exposure of a patient to ionizing radiation. Suchapplications require high radiation absorption characteristics for thedetector substrate of the imaging device. Such high radiation absorptioncharacteristics can be provided by materials using high Z element suchas CdZnTe or CdTe.

Furthermore, various medical applications require high spatialresolution. For example, mammography requires the ability to observemicrocalcifications which can be under 100 microns or even under 50microns in size. The stringent requirements imposed on imaging devicesrequire the use of small resolution elements (pixel cells), with a largearrays of such cells being needed to generate an image of a useful size.

An important step in the fabrication of such imaging devices is thebonding of the semiconductor substrate to the readout substrate, or moreprecisely, the bonding of detector cells to corresponding readout cellsin a one-to-one correspondence.

A semiconductor pixel imaging device is disclosed in commonly assignedand copending U.S. patent application Ser. No. 08/454,789, the entiretyof which is incorporated by reference herein. As mentioned in theprevious paragraph, a significant aspect of this technology is thebonding of the semiconductor substrate to the readout substrate.

Typically, prior art hybrid imaging devices such as those described inU.S. Pat. No. 5,245,191, EP-A-0 571, 135, and EP-A-0 577 187 employindium bumps for bump-bonding a detector substrate to a readoutsubstrate.

Indium bumps are grown on the detector metal contacts (defining thecells) and on the readout cells using evaporation. Subsequently, the twodifferent parts are brought together, aligned, and the correspondingbumps are merged. This is also termed flip-chip joining. This coldwelding technique is achieved by heating the substrates at 70-120° C.and applying mechanical pressure. For detectors comprising heatsensitive materials such as cadmium zinc telluride (CdZnTe) and cadmiumtelluride (CdTe) the use of indium bumps is advantageous in that theprocess can be carried out at a low temperature. The temperatures neededfor indium bump-bonding, typically 70-120° C., fall within an acceptablerange for materials such as CdZnTe and CdTe.

However, during the development of imaging devices using indiumbump-bonding, non-uniform detector response has been observed near thedetector edges. A plausible explanation is that indium is escaping tothe detector edges thus creating undesirable contact between edge metalcontacts (edge pixels) and the detector edge.

The present invention seeks to mitigate the problems of the prior art.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided animaging device for imaging radiation, the imaging device comprising asemiconductor substrate including an array of detector cells whichgenerate charge in response to incident radiation and a correspondingreadout semiconductor substrate including an array of readout cells,said readout cells being connected to corresponding detector cells bymeans of low temperature solder bumps comprised of lead-tin based solderwith a melting point below that of eutectic lead-tin solder (183° C.).

An imaging device according to the invention provides improved accuracyand uniformity as a result of the bonding process employed. Inparticular, the method is self-aligning in that, during heating of thestructure, surface tension of the melting bump forces the detector andreadout substrates to align with one another.

Although the use of solder for joining circuits together is well knownin the electronics arts, the normal type of solder, the eutectic form ofwhich is formed from 60 percent tin (Sn) and 40 percent lead (Pb) byweight, requires the use of temperatures of 183° C. or more. Suchtemperatures, even if applied for only a short time, damage sensitivedetector substrates made of materials such as CdZnTe and CdTe.

Surprisingly, through the use of low temperature solder in accordancewith the invention, the disadvantages of indium bump bonding can beavoided without causing damage to the detector substrate, even if it ismade of CdZnTe or CdTe, which would be the case were conventional solderto be used.

Moreover, the use of low temperature solder avoids the need to formbumps on both the detector and readout substrates, which provides foreconomies of manufacture as well as improved performance andreliability. This avoids a further disadvantage of the prior art, whichrequires the application of indium bumps to both substrates.

Preferably the solder bumps comprise solder having a melting point under180° C., more preferably below 120° C., and yet more preferably below100° C. Preferably, the solder comprises an alloy of bismuth (Bi), lead(Pb) and tin (Sn).

A preferred alloy which gives a low melting point of the order of 90° C.comprises approximately 52 weight percent of Bi, approximately 32 weightpercent of Pb and approximately 16 weight percent Sn.

As mentioned above, preferred embodiments employ a detector substrate ofCdZnTe or CdTe because of the high energy radiation absorptioncharacteristics of those materials. However, it will be appreciated thatthe invention could be used with other detector substrate materials,even if they are not as temperature sensitive as CdZnTe or CdTe. Thereadout chip can, for example, be a CMOS chip.

The invention also provides an imaging system comprising at least oneimaging device as described above.

An imaging device as described above finds particular application formedical diagnosis and/or for non-destructive testing.

In accordance with another aspect of the invention, there is provided amethod of manufacturing an imaging device having an array of image cellsfor imaging radiation, the imaging device comprising a detectorsemiconductor substrate including an array of detector cells forgenerating charge in response to incident radiation and a readoutsemiconductor substrate including an array of corresponding readoutcells, the method comprising steps of: applying low temperature solderbumps to one of the substrates at positions corresponding to the imagecells; aligning respective readout and detector cells to each other; andconnecting the detector and readout cells by the application of heat tothe low temperature solder bumps, low temperature solder preferablybeing a lead-tin based solder with a melting point below that ofeutectic lead-tin solder.

Preferably the solder bumps are applied to the readout systems only, butthey may alternatively or additionally be applied to the detectorsubstrate.

Preferably, to assist in obtaining an accurate alloy composition for thelow temperature solder, and thereby to ensure an accurate meltingtemperature for the low temperature solder, the step of applying lowtemperature solder bumps comprises applying constituent elements of thelow temperature solder in required proportions at positions for thesolder bumps and then applying heat to reflow the constituent elementsto form the solder bumps.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be described hereinafter,with reference to the accompanying drawings in which:

FIG. 1 is a schematic overview of an imaging system for high energyradiation imaging.

FIG. 2 is a schematic cross sectional diagram of an example of imagingdevice in accordance with the invention.

FIGS. 3A-3D are schematic diagrams illustrating a method ofmanufacturing such an imaging device in accordance with invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of an example of an imaging system10 including an embodiment of an imaging device in accordance with theinvention.

This application relates to radiation imaging of an object 12 subjectedto radiation 14. In one application of the disclosed invention, theradiation may, for example, be X-ray radiation and the object 12 may,for example, be a part of a human body.

The imaging device 16 comprises a plurality of pixel cells 18. Theimaging device directly detects high energy incident radiation (e.g.,radiation having an energy level greater than 1 kev) such as X-rays,γ-rays, β-rays or α-rays. The imaging device is configured on twosubstrates, one with an array of pixel detectors 19 and one with anarray of readout circuits 20, the substrates being mechanicallyconnected to each other by low temperature solder bumps comprised oflead-tin based solder with a melting point below that of eutecticlead-tin solder.

Control electronics 24 includes processing and control circuitry forcontrolling the operation of the imaging device, or an array of imagingdevices. The control electronics 24 enables the readout circuits 20associated with individual pixel cells 18 to be addressed (e.g.,scanned) for reading out charge from the readout circuits 20 at theindividual pixel cells 18. The charge readout is supplied to Analog toDigital Converters (ADCs) for digitization and Data Reduction Processors(DRPs) for processing the digital signal.

The processing which is performed by the DRPs can involve discriminatingsignals which do not satisfy certain conditions such as a minimum energylevel. This is particularly useful when each readout signal correspondsto a single incident radiation event. If the energy corresponding to themeasured signal is less than that to be expected for the radiation used,it can be concluded that the reduced charge value stored results fromscattering effects. In such a case, the measurement can be discardedwith a resulting improvement in image resolution.

The control electronics 24 is further interfaced via a path representedschematically by the arrow 26 to an image processor 28. The imageprocessor 28 includes data storage in which it stores digital valuesrepresentative of the charge values read from each pixel cell along withthe position of the pixel cell 18 concerned. The image processor 28builds up an image for display. It then reads the values stored for theselected pixel positions to cause a representation of the data to bedisplayed on a display 52 via a path represented schematically by thearrow 50. The data can, of course, be printed rather than, or inaddition to being displayed and can be subjected to further processingoperations. Input devices 56, for example, a keyboard and/or othertypical computer input devices, are provided for controlling the imageprocessor 28 and the display 52 as represented by the arrows 54 and 58.

FIG. 2 is a schematic cross section of part of an imaging device 16. Inthis example, the imaging device 16 comprises an image detectorsubstrate 30 connected to an image circuit substrate 32 by means ofsolder bumps 34. A pixel detector 19 of each pixel cell 18 is defined onthe detector substrate 30 by a continuous electrode 36 which applies abiasing voltage and pixel location electrodes (contact pads) 38 todefined a detection zone for the pixel cell 18. Corresponding pixelcircuits 20 on the image circuit substrate 32 are defined at locationscorresponding to the electrodes 38 (i.e. to the pixel detectors 19).Electrodes (contact pads) 40 for the pixel circuits 20 are electricallyconnected to the corresponding electrodes 38 by the solder bumps 34. Inthis manner, when charge is generated in a pixel detector 19 in responseto incident radiation, this charge is passed via the solder bumps 34 tothe corresponding pixel circuit 20.

Thus, each pixel cell 18 of the imaging device 16 is in effect definedon the substrate by electrodes (not shown) which apply a biasing voltageto define a detection zone (i.e., the pixel detector 19) for the pixelcell 18. Corresponding readout circuits on the readout substrate cancomprise, for example, active pixel circuits 20 as described in commonlyassigned and copending U.S. patent application Ser. No. 08/454,789, theentirety of which is incorporated by reference herein. The pixeldetectors 19 are formed with a detection zone such that, when a photonis photo-absorbed in the semiconductor substrate 16 at a pixel cell 18creating an electric charge or when a charged radiation ionizes thedetection zone of the semiconductor substrate 16 at a pixel cell 18, anelectric pulse flows from the semiconductor 16 at a pixel cell 18, anelectric pulse flows from the semi-conductor detection zone to thereadout circuit 20 for that pixel cell 18 through the solder bump 34 forthat pixel cell.

In order to provide efficient charge absorption for X-rays and otherhigh energy radiation typically having energies in excess of 1 keV, theuse of high absorption semiconductor materials for the detectorsubstrate is desirable, for example, CdZnTe or CdTe. In this case, lowtemperature processes used during manufacture avoid damaging thetemperature sensitive substrate.

Thus, through the use of low temperature soldering (under 180°)sensitive materials such as CdZnTe or CdTe can be used without impairingthe characteristics of the detector substrate.

An example of an imaging device in accordance with the invention,therefore comprises a semiconductor substrate and a readout substrate,the substrates comprising detecting and readout cells respectively, eachdetecting cell being connected to a corresponding (one-to-onecorrespondence) readout cell with low temperature solder bumps.

By way of example, monolithic detectors of dimensions 12.2×4.2 mm²(41,000 pixels of 35 microns size) and 18.9×9.6 mm² (130,000 pixels of35 microns size) connected to a CMOS chip via low temperature solderbumps may be constructed. However, the actual size of the pixel circuitand the pixel detector will depend on the application for which theimaging device is intended, and the circuit technology used.

Such an imaging device will then exhibit the necessary uniformperformance over a large number of bonded cells thus meeting thecriteria (high absorption efficiency, high spatial resolution) for usein medical diagnosis, for example mammography, dental imaging, chestX-rays, conventional X-rays, fluoroscopy, computerized tomography,nuclear medicine and non-destructive testing.

Low temperature solder bumps may be as small as 5 microns in diameterbut may be larger. A soldering material with low melting point will be asuitable low temperature solder. A low temperature solder is a solderwhich can be melted at a temperature which will mitigate or preventdamage or deterioration of a temperature sensitive detector substratesuch as CdZnTe or CdTe. A low temperature solder has a melting point ofpreferably less than 180° C., more preferably less than 120° C. and yetmore preferably less than 100° C.

One example of such a low temperature solder material is a ternarybismuth-lead-tin (BiPbSn) alloy. The melting point of a eutectic (52 wt% Bi, 32 wt % Pb, 16 wt % Sn) alloy is, for example, under 100° at about90° C. The percentages of the composition are each approximate. Thealloy may be made solely of the three elements mentioned inapproximately the proportions indicated to a total of 100 wt %. However,the alloy composition may be varied to optimize wetting, melting pointand/or thermal expansion on solidification. For example, the proportionsof the component elements may be varied and/or other component elementsmay be chosen for addition to or substitution for the elementsmentioned.

FIG. 3 is a schematic representation of a method of manufacturing animaging device as described above. FIG. 3A represents a step ofproviding a readout substrate 32 with an array of contact pads 40 forconnections to corresponding contact pads 38 on a detector substrate 30(FIG. 3C).

FIG. 3A represents the provision of solder bumps 34 on the contact pads40. The solder bumps can be formed, for example, by vacuum evaporationor electroplating for depositing the metal alloy solder material onrespective contact pads. A metal or photoresist mask may be used. Toattain an accurate alloy composition, each constituent metal may bedeposited separately but then, prior to joining, the structure issubjected to a process step in which the bumps are reflowed, (subjectedto a temperature higher than the alloy's melting point) thushomogenizing the bump composition at each contact pad position. It isnot necessary to exceed significantly the melting point of the alloy, inorder to reflow the layered "sandwich" structure.

In a preferred embodiment of the invention, the bump is deposited on thereadout chip side only as shown in FIG. 3B so as to spare the detectorfrom any harmful deposition and for economy of tasks (avoiding growingbumps on the detector substrate).

Alternatively, task economy could also be achieved by depositing thesolder bumps 34 on the detector substrate 30 (FIG. 3C) instead, althoughthis would increase the risk of possible damage to the detectorsubstrate.

As a further alternative, bumps can be grown on both the readoutsubstrate 32 and on the detector substrate 30 if a suitable bump volumecannot be attained otherwise.

A solderable (solder wettable) pad can be formed underneath the solderbump. This pad can be deposited prior to bump deposition using the samemask. It is not necessary to use the same technique for depositing boththe bump and the under-bump metallurgy. An additional advantage providedby low temperature solder is that it allows for thinner under-bumpmetallurgies, as well as providing the choice of using otherwiseunusable metals, as the rate at which the under-bump metallurgydissolves into the bump is proportional to temperature.

Guard rings, also made of solder, and in addition to their electricalfunction, may be used around the pixel array hermetically to seal thepixel area solder joints from external atmosphere. Dams and/or shields,for electrical and/or mechanical purposes, may also be constructed.

The bumps need not all be of the same size. A small number of relativelylarge bumps may be used to aid the self-alignment of the main pixelarray with a large number of relatively small bumps.

Once the solder bumps are formed, then the readout substrate 34 (FIG.3B) is flip-chip joined to the detector substrate (FIG. 3C), asrepresented by the arrow 50 with the controlled application of heat at atemperature and for a time sufficient to "soften" the solder bumpssufficiently to enable connection of the semiconductor substrates, butnot sufficient to cause damage to the semiconductor substrates. Suchheating can take place over a period of time varying from, for example,a few seconds to several minutes. In this manner joining of therespective contact pads 38, 40 on the detector substrate 30 and thereadout substrate 32, respectively, can be achieved.

FIG. 3D represents one corner of the joined hybrids imaging device 16.

Thus a semiconductor imaging device, for use in, for example, medicaldiagnosis and non-destructive testing, has been described. Thesemiconductor imaging device includes a radiation detector semiconductorsubstrate and a readout substrate connected to the detector by means oflow temperature solder bumps. A low temperature solder should have amelting point under about 180° C., preferably less than 150° C., morepreferably less than 120° C. and yet more preferably less than 100° C.Examples of such low temperature solders are provided by bismuth-basedalloys of lead and tin, for example the eutectic alloy, which iscomposed of approximately 52 wt % Bi, approximately 32 wt %Pb andapproximately 16 wt % Sn to 100 wt % and has a melting point under 100°C.

An imaging device in accordance with the inventions disclosed herein canbe used in applications such as medical diagnosis, for example formammography, dental imaging, chest X-rays, fluoroscopy, computerizedtomography, nuclear medicine and so on. An imaging device in accordancewith these inventions can also be used in applications such asnon-destructive testing.

The foregoing is a detailed description of particular embodiments of theinvention. The invention embraces all alternatives, modifications andvariations that fall within the letter and spirit of the claims, as wellas all equivalents of the claimed subject matter. Although particularexamples and combinations of materials, configurations and methods ofmanufacture for other embodiments of the invention have been described,other examples, combinations, configurations and methods and otherembodiments are possible within the spirit and scope of the invention.

What is claimed is:
 1. An imaging device for imaging radiation, saidimaging device comprising a semiconductor substrate including an arrayof detector cells which generate charge in response to incidentradiation and a corresponding readout semiconductor substrate includingan array of readout cells, said readout cells being connected tocorresponding detector cells by low temperature solder bumps, whereinsaid low temperature solder bumps comprise lead-tin based solder havinga melting point below that of eutectic lead-tin solder.
 2. The imagingdevice of claim 1 wherein said solder bumps comprise solder having amelting point under 180° C.
 3. The imaging device of claim 1 whereinsaid solder bumps comprise solder having a melting point under 120° C.4. The imaging device of claim 1 wherein said solder bumps comprisesolder having a melting point under 100° C.
 5. The imaging device ofclaim 1 wherein said detector substrate comprises CdTe.
 6. The imagingdevice of claim 1 wherein said solder bumps comprise solder includingBi, Pb, and Sn.
 7. The imaging device of claim 1 wherein said solderbumps comprise solder comprised of approximately 52 percent Bi,approximately 32 percent Pb and approximately 16 percent Sn.
 8. Theimaging device of claim 1 wherein said solder bumps comprise soldercomprised of Bi and Pb and between 1 and 65 percent Sn.
 9. The imagingdevice of claim 1 wherein said solder bumps comprise solder comprised ofBi and Sn and between 1 and 75 percent Pb.
 10. The imaging device ofclaim 1 wherein said solder bumps comprise solder comprised of Pb and Snand between 1 and 75 percent Bi.
 11. The imaging device of claim 1wherein said solder bumps comprise a solder alloy including at least oneof In, Cd, Ga, Zn, Ag or Au.
 12. The imaging device of claim 1 whereinsaid detector substrate comprises CdZnTe.
 13. An imaging systemcomprising:an imaging device of imaging radiation, said imaging devicecomprising an array of detectors which generate charge in response toincident radiation and an array for readout devices connected tocorresponding elements of said array of detectors by low temperaturesolder bumps comprising lead-tin based solder having a melting pointbelow that of eutectic lead-tin solder; control electronics operablycoupled to said imaging device for controlling reading by said readoutdevices and processing output from said readout devices; and an imageprocessor responsive to processed output from said control electronicsfor generating an image therefrom.
 14. The imaging system of claim 13wherein said control electronics comprise analog to digital converters.15. The imaging system of claim 13 wherein each of said detectors is adetector cell on a semiconductor substrate.
 16. The imaging system ofclaim 13 wherein each of said readout devices is a readout cell on anext semiconductor substrate.
 17. The imaging system of claim 14 whereinsaid control electronics further comprise data reduction processors. 18.A method of manufacturing an imaging device comprising a detectorsemiconductor substrate including an array of detector cells forgenerating charge in response to incident radiation and a readoutsemiconductor substrate including an array of readout cells, one of saiddetector cells and one of said readout cells forming an image cell, saidmethod comprising:applying low temperature solder bumps comprisinglead-tin based solder having a melting point below that of eutecticlead-tin solder to one of said substrates at positions corresponding tosaid image cells; aligning respective readout and detector cells to eachother; and connecting said detector and said readout cells by theapplication of heat to said low temperature solder bumps.
 19. The methodof claim 18 wherein said solder is an alloy having a plurality ofconstituent elements and said step of applying low temperature solderbumps comprises:applying constituent elements of said low temperaturesolder in required proportions at positions for said solder bumps; andapplying heat to reflow said constituent elements to form said solderbumps.
 20. The method of claim 18 wherein said solder bumps are appliedto said readout substrate at positions corresponding to said readoutcells.
 21. The method of claim 18 wherein said solder bumps are appliedto said readout substrate at positions corresponding to said readoutcells and to said detector substrate at positions corresponding to saiddetector cells.
 22. The method of claim 18 wherein said solder bumpscomprise solder having a melting point under 180° C.
 23. The method ofclaim 18 wherein said solder bumps comprise solder having a meltingpoint under 120° C.
 24. The method of claim 18 wherein said solder bumpscomprise solder having a melting point under 100° C.
 25. The method ofclaim 18 wherein said solder bumps comprise a solder alloy of Bi, Pb,and Sn.
 26. The method of claim 18 wherein said solder bumps comprise asolder alloy of approximately 52 percent Bi, approximately 32 percentPb, and approximately 16 percent Sn.
 27. The method of claim 18 whereinsaid solder bumps comprise solder comprised of Bi and Pb and between 1and 65 percent Sn.
 28. The method of claim 18 wherein said solder bumpscomprise solder comprised of Bi and Sn and between 1 and 75 percent Pb.29. The method of claim 18 wherein said solder bumps comprise soldercomprised of Pb and Sn and between 1 and 75 percent Bi.
 30. The methodof claim 18 wherein said solder bumps comprise a solder alloy includingat least one of In, Cd, Ga, Zn, Ag or Au.